One guideline when using artificial obstacles is to place them at a distance of what from your position?

Placing artificial obstacles beyond hand grenade range is an effective strategy because it keeps them safely out of reach of potential threats. This distance ensures that if an enemy were to use hand grenades or similar explosives, they would not be able to easily destroy or neutralize those obstacles from a direct assault position. By positioning obstacles outside this range, you enhance their effectiveness in slowing down or deterring an enemy advance, while also maintaining a safer zone for your own personnel. This approach balances tactical defense with the need to protect one's own forces from retaliatory actions.

The other options, while they may have merits in different contexts, do not provide the same level of security against explosive threats, making beyond hand grenade range the most tactical choice in this scenario.